The following background description constitutes a description of the background to the present invention, which does not, however, necessarily constitute prior art.
For motor vehicles, such as cars, trucks and buses, the cost of fuel constitutes a significant expense for the owner or user of the vehicle. For a hauling company, for example, the main expenditure items for the day to day operation of a vehicle consist, apart from the cost of acquisition of the vehicle, of the vehicle driver's salary, costs of repair and maintenance, and fuel for driving the vehicle. The fuel cost may impact the profitability for the hauling company to a great extent. Therefore, a number of different systems have been developed to reduce fuel consumption, such as fuel-efficient engines and fuel-economical cruise controls.
One objective of cruise control systems is to achieve an even, predetermined speed. This is either achieved by adjusting the engine torque in order to avoid retardation, or by application of braking action on downhill slopes where the vehicle accelerates by virtue of its own weight. An overall objective of the cruise control is to achieve a comfortable drive and increased comfort for the driver of the motor vehicle.
A driver of a motor vehicle with a cruise control usually selects a set speed vset. The set speed vset is the speed which the driver wishes the motor vehicle to maintain on a level road. The cruise control then provides the engine system in the vehicle with a reference speed vref, where the reference speed vref is used to control the engine. The set speed vset may thus be seen as an input signal to the cruise control, while the reference speed vref may be seen as an output signal from the cruise control, which is used to control the engine, providing control of the vehicle's actual speed vact.
Traditional cruise controls (Cruise Control; CC) maintain a constant reference speed vref, which corresponds to the set speed vset set by the driver. The value of the reference speed vref is here changed only when the user himself adjusts the set speed vset while driving.
Today there are also cruise controls, so-called economical cruise controls, such as Ecocruise controls and similar cruise controls, which try to estimate the current driving resistance and also have knowledge about the historical driving resistance. An experienced driver who drives a motor vehicle without a cruise control may reduce fuel consumption by adjusting his driving to the characteristics of the road ahead, so that unnecessary braking and/or fuel-consuming acceleration may be avoided. In a further development of these economical cruise controls, the ambition is to mimic the experienced driver's adjustment of driving the motor vehicle based on knowledge about the road ahead, so that fuel consumption may be kept at as low a level as possible, since this impacts the profitability for the owner of the motor vehicle, such as a haulage company or similar, to a great extent.
One example of such a further development of an economical cruise control is a “Look Ahead” cruise control (LACC), i.e. a strategic cruise control using knowledge about road sections ahead, i.e. knowledge about the appearance of the road ahead, in order to determine the appearance of the reference speed vref. Here, the reference speed vref is thus permitted, within a speed interval vmin-vmax, to differ from the set speed selected by the driver vset in order to achieve more fuel-efficient driving.
Knowledge about the road section ahead may consist of knowledge about prevailing topography, curvature, traffic situation, road works, traffic intensity and road condition. Further, the knowledge may consist of a speed limit for the road section ahead, and/or of a road sign in connection with the road. These pieces of knowledge may be obtained through positioning information, such as GPS information (Global Positioning System information), map information and/or topographical map information, weather reports, information communicated between different vehicles and information communicated via radio. This knowledge may be used in many ways. For example, knowledge about a future speed limit for the road may be used to achieve fuel-efficient reductions of speed before a future lower speed limit. Similarly, knowledge about a road sign, with information about e.g. a future roundabout or junction, may also be used to brake in a fuel efficient manner before the roundabout or junction. By basing cruise control on positioning information combined with topographical map information, wrong decisions, depending on a driver's inaccurate judgement about a road gradient, may be avoided.
A LACC cruise control may allow increasing a reference speed vref before a steep uphill slope to a level which is above the level for the set speed vset, since the motor vehicle is expected to lose speed on the steep uphill slope, due to a high train weight in relation to the vehicle's engine performance. This means that time is saved, meaning that the LACC cruise control may be seen as a cruise control improving driveability. Similarly, the LACC cruise control allows the reference speed vref to be reduced to a level which is below the set speed vset before a steep downhill slope, since the motor vehicle is expected to accelerate on the steep downhill slope due to the high train weight. The idea here is, that it is more fuel-economical to use the motor vehicle's acceleration due to its own weight on the downhill slope, than to first accelerate before the downhill slope and then brake down the hill. The LACC cruise control may thus reduce fuel consumption with a substantially unchanged driving time.
There are also cruise controls which, based on a current driving resistance, decide how the motor vehicle's speed should vary. In these cruise controls the reference speed vref may be allowed to diverge from the set speed vset, within a speed interval vmin-vmax, based on at least one characteristic of the driving resistance, such as its size and/or appearance over time.
There are also cruise controls which use radar and/or cameras to obtain information about vehicles ahead. Based on this information, the cruise control may control the vehicle's actual speed vact, within a speed interval vmin-vmax, so that e.g. a substantially constant distance is maintained to the vehicle in front. In the event of e.g. downhill slopes, or in situations where the vehicle must reduce its actual speed, fuel savings have historically also been made through a reduced request of positive engine torque or with the help of dragging. The reduced request of positive engine torque entails that the force driving in the direction of travel, emitted by the combustion engine via the driving wheels, is reduced, e.g. through reduced fuel injection in the engine, which reduces the fuel consumption.
Dragging means driving the vehicle with a closed driveline, i.e. with the combustion engine connected to the vehicle's driving wheels, while at the same time the fuel injection to the combustion engine is closed. One advantage with this type of measure is that, since the fuel injection to the combustion engine is closed, the combustion engine's consumption is equal to zero. The measure does, however, also entail that the combustion engine will be driven by the vehicle's driving wheels via the driveline, thus achieving so-called “dragging”, so that the combustion engine's inertia gives rise to a braking action, i.e. the vehicle is engine-braked.
A reduction of the requested engine torque and dragging does reduce fuel consumption, but this reduction is not always optimized, partly because the reduced engine torque nevertheless often consumes more fuel than necessary, and partly because the dragging also adds an engine-braking of the vehicle, which is not fuel economical.
When the vehicle is driven, the gear selection has a great impact on the fuel consumption, since the engine speed is directly dependent on this gear selection. Thus, the fuel consumption for an actual speed vact is generally lower for a higher gear compared to a lower gear. An upshift means that the forces that act against the vehicle's movement are reduced, since the force for the engine friction Feng becomes smaller as the engine speed is reduced. Thus an upshift may reduce fuel consumption, also since the resistance against the vehicle is reduced.
There are therefore cruise controls comprising a function trying to achieve that as high a gear as possible is used, at the same time as an actual speed vact for the vehicle within a speed interval vmin-vmax is obtained.
In this document, an upshift means that a higher potential gear in the gearbox is selected, where this higher gear is physical, i.e. it constitutes one of the gears in the gearbox. This higher gear also means that the engine works at a lower engine speed if the actual speed vact is maintained.
Another way of reducing fuel consumption is to let the vehicle coast, as described below, by either using a neutral gear in the gearbox or opening the clutch. By using coasting, even lower fuel consumption is achieved than with dragging, since engine-braking is eliminated while the engine speed is reduced to a minimum. Coasting may be carried out with the engine working or switched off.
Coasting here means that the vehicle's engine is disconnected from the engine's driving wheels, so that the driveline is opened. This disconnection of the driving wheels from the engine, also called opening of the driveline, may be achieved by putting the gearbox in a neutral gear or by opening the clutch. In other words, during coasting essentially no power is transmitted from the engine to the driving wheels through the gearbox. In this document, the concept of coasting also includes that one of several engines in a vehicle, e.g. in a hybrid vehicle, are disconnected from the driving wheels. For example, the concept of coasting comprises a manner of driving in a hybrid vehicle, where a combustion engine is disconnected from the driving wheels and where an electric engine then still may transmit power to the driving wheels.
Coasting means that the forces that act against the vehicle's movement are significantly reduced, since the force for the engine friction Feng then shrinks to a value substantially equal to zero (0). Therefore, coasting may significantly reduce fuel consumption through this reduction of the resistance against the vehicle. In certain cases of coasting idling fuel must, however, be provided to the engine so that it does not stop, whereas the engine may be permitted to stop in other cases.
This means, that from a fuel perspective it is often more advantageous to drive the vehicle with an open driveline, i.e. coasting, than with dragging, i.e. when the driveline is closed while simultaneously the fuel injection to the engine is closed. The reason for this is that the limited amount of fuel, which is required to keep the combustion engine in operation when the combustion engine is disconnected, is compensated for by the fact that the vehicle may continue with a disconnected combustion engine a longer distance, e.g. after passing a downhill slope. This depends among others on the vehicle achieving a higher speed on e.g. the downhill slope, when driven with a disconnected combustion engine, compared to driving the vehicle with a closed driveline without fuel injection. There are also variants of coasting for which the combustion engine is switched off during coasting, no fuel injection being required.
In addition, when coasting, the force which counteracts the vehicle being driven will be lower when the vehicle's combustion engine is disconnected from the driving shaft, since there is no engine-braking force that counteracts the progress of the vehicle. This means that the vehicle will retard more slowly, e.g. when the vehicle reaches the end of a downhill slope, which in turn means that coasting may often be used over a relatively long distance after e.g. the end of a downhill slope. Thus a significant reduction in fuel consumption is obtained.
There are therefore cruise controls comprising a function trying to achieve that coasting is used, if it is possible to obtain an actual speed vact within a speed interval vmin-vmax thereby. Such cruise controls may increase the actual speed vact before road sections for which coasting is expected to be possible.